Fuel cell plants require often the supply of hydrogen as fuel source and accordingly a reforming reactor is normally integrated in fuel cell plants. The reforming reactor converts a suitable hydrocarbon feedstock acting as energy carrier, such as methane, liquid petroleum gas, gasoline, diesel or methanol, into a hydrogen rich gas, which then may be passed through a hydrogen-enrichment unit before entering a fuel cell assembly. Compact fuel cell power plants may today provide about 20 kW of power and even more, for instance up to 50 kW, thereby promoting a wide range of applications. One such application is the use of compact fuel cell plants in the automotive industry.
For widespread application, methanol is still regarded as the best hydrocarbon feedstock for the production of hydrogen-rich gas, not only in connection with fuel cell plants but also for application in small plants in other industrial fields. Roughly, methanol is particularly suitable where the demand for hydrogen is the range 50-500 Nm3/h, which is typical for small plants. For a hydrogen demand of above 500 Nm3/h a hydrocarbon feedstock such as natural gas is often more expedient. Below 50 Nm3/h electrolysis or bottled hydrogen is normally more expedient.
Reactors for the reforming of fuel gases, particularly methanol, and which are used in fuel cell plants are known in the art. Düsterwald et al. disclose in Chem. Eng. Technol. 20 (1997) 617-623 a methanol steam reformer consisting of four reactor tubes that are individually balanced. Each reactor tube consists of two stainless tubes arranged concentrically with catalyst filling the inner tube and in which the heat needed for the endothermic reaction of a methanol-water mixture is provided by condensing steam that flows in the gap between the tubes. It is also known from U.S. Pat. No. 4,861,347 to oxidise a raw fuel, such as methanol, in order to obtain an exothermic reaction, whereby the heat generated by this reaction is used for the endothermic reforming reaction of the hydrocarbon feedstock, which is normally a mixture of methanol and water. The heat is transferred from the combustion section of the reactor to its reforming section by means of heat tubes through which a hot flue gas from the combustion section is passed or as in JP-A-63248702 by means of heat pipes arranged in the reactor. As a result, the heat generated in the combustion system can be evenly distributed to the rest of the reactor, whereby a uniform temperature distribution is obtained.
Often the heat transfer system in the reforming reactor is not rapid enough to achieve the desired operating temperature after changes in process conditions, such as after sudden load changes or during start-ups and shut-downs, especially when separate heat pipes are provided in the reforming reactor. Normally a number of more or less sequential steps are required for the start-up of the reforming reactor, resulting in a procedure that may be significantly tedious and time-consuming.
In the particular field of fuel cells, the advent of fuel cells with increased power, for instance of up to 20 kW or even more, for instance up to 50 kW has resulted in a need for providing a plurality of catalyst tubes in a single reforming reactor. This in turn imposes more demands in reactor design in terms of i.e. compactness, better temperature distribution and thermal efficiency. In particular, the provision of a uniform temperature distribution by which all catalyst tubes inside the reactor are heated to the same temperature becomes more difficult to achieve when the heating required in reforming has to be provided by means of a single burner in the reactor.
In addition, the catalyst within the catalyst tubes may often be not evenly distributed so that the catalyst may for instance be better packed in some tubes than others. This may create undesired variation in temperature conditions across the catalyst tubes.